A 5,000-year-old toy design may hold promise for changing how blood-related conditions such as anemia, HIV, and malaria are diagnosed in the developing world. A new version of the toy, a device in which circular motion is facilitated by two twisting strings, has been found to separate blood as efficiently as certain commercial centrifuges. Called a “paperfuge”, it can be produced for about 20 cents, since it is made from just paper, string, and glue. Traditional centrifuges cost hundreds or thousands of dollars. To be used for medical purposes, the device will have to pass a number of regulatory obstacles.

Modern centrifuges, an essential part of modern medical laboratories, can cost up to 6000 dollars for some versions. This has prevented doctors in the developing world from gaining access to the devices, which are essential for separating blood and other materials. Furthermore, even the labs that do have access to centrifuges do not always have consistent electricity to keep them running. Instead, alternatives have been created without the need for electricity, using simple machines built using egg beaters and salad spinners. Unfortunately, these devices cannot match the speed of modern commercial centrifuges, which can spit at more than 100,000 revolutions per minute.

A team of researchers led by Manu Prakash, a bioengineer at Stanford, looked into a range of spinning toys such as yo-yos and tops, gauging their speeds with high-speed cameras. They even hired a circus performer to get the best performance out of the toys.

Prakash said the team “amassed a graveyard of spinning toys” that failed to meet their needs. Prakash is also known for creating an inexpensive paper microscope he calls the Foldscope.

One toy held more promise than the others – a whirligig, also called a buzzer, bullroarer or a zumbado. The toy goes back thousands of years, found from East Asia to Latin America. Prakash himself knew the toy as a “button on a string” growing up in India.

However, how the toy worked was still largely unknown. Prakash and his team explored this, using computational modeling. They found that the clumped coils of string stored energy facilitated the rapid spinning.

Prakash said:

“The way the strings wind and unwind, they exploit a really interesting principle of supercoiling. These supercoils … let it go far beyond its geometrical limits.”

The team then spent 6 months creating a version of the toy that maximizes rotational speed. Winding up with a prototype that uses sticks attached by strings to a paper disk that contains small tubes of blood. Pulling back and forth on the sticks winds and unwinds the strings, spinning the tubes of blood. The prototype was found to spin at up to 125,000 revolutions per minute – faster than many commercial centrifuges. Comparable to modern centrifuges, it was able to separate plasma from blood in under a minute and a half.

The team reported their findings in Nature Biomedical Engineering.

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